1. Field of the Invention
The invention relates to the field of optical filters, and in particular to filter arrangements using fiber Bragg grating elements.
2. Description of the Related Art
It is known to form phase gratings along the core of an optical waveguide, for example, by the application of intense beams of ultraviolet light transverse to the axis of the core at selected angles of incidence and the compliments thereto. In particular, an intra-core fiber Bragg grating (FBG) element can be formed in a Ge-doped fiber using side illumination to form an ultraviolet (UV) interference pattern, as disclosed in, for example, G. Meltz, W. W. Morey, and W. H. Glenn, "Formation of Bragg gratings in optical fiber by a Transverse Holographic Method," Optics Letters, 14, pp 823-825, 1989.
In this known process, the gratings are formed by transverse irradiation with a particular wavelength of light in the ultraviolet absorption band of the core material, establishing a first order absorption process by which gratings characterized by a particular spacing and wavelength can be formed by illuminating the core from the side with two coplanar, coherent beams incident at selected and complementary angles thereto with respect to the axis of the core. The grating period is selected by varying the selected angles of incidence. Thus, a permanent change in the refractive index is induced in a predetermined region of the core, creating, in effect, a phase grating for affecting light in the core at selected wavelengths. See U.S. Pat. Nos. 4,725,110 and 4,807,950.
The Bragg wavelength of these elements, that is, the wavelength of incident light which will be reflected back, depends on the spacing of the lines in the fiber Bragg grating, i.e., the periodicity of the UV interference pattern and the resultant core-index of refraction modulation in the fiber, as well as the effective index (combination of core index and cladding index) of the guided optical mode.
These intra-core fiber Bragg grating elements are suitable as optical wavelength sensor elements, e.g., strain or temperature sensors, since the Bragg wavelength will be changed by a change in the temperature and/or the strain on the fiber which modifies the spacing and index of refraction of the grating.
An additional known use of these fiber Bragg grating elements is in wavelength filtering, since the basic device acts as a narrow-band band-reject or notch-filter for light transmitted through the element, and as a selective narrow-band bandpass filter for light reflected from the element. By using a pair of identical grating reflectors, a Fabry-Perot interferometer can be formed as a sensor element in the fiber, with an enhanced sensitivity, see W. W. Morey, J. R. Dunphy and G. Meltz, "Multiplexed Fiber Bragg Grating Sensors", Proc. `Distributed and Multiplexed Fiber Optic Sensors`, SPIE vol. 1586, pp. 216-224, Boston, Sep., 1991.
Various methods of wavelength stabilization and tuning of lasers, in particular doped-fiber ring laser configurations, are also known. See, for example, P. R. Morkel, G. J. Cowle, and D. N. Payne, "Travelling-Wave Erbium Fiber Ring Laser with 60 kHz Linewidth", Electron Letter, 26, pp. 632-634, 1990; K. Iwatsuki, H. Okamura and M. Saruwatari, "Wavelength-Tunable Single-Frequency and Single Polarization Er-Doped Fiber Ring Laser with 1.4 kHz Linewidth", Electron. Letter, 26, pp. 2033-2035, 1990; and N. Park, J. W. Dawson, K. J. Vahala and C. Miller, "All-Fiber, Low Threshold, Widely Tunable Single Frequency, Erbium-Doped Fiber Ring Laser with a Tandem Fiber Fabry Perot Filter", Appl. Phys. Letter, 59, pp. 2369-2371, 1991.
Ring laser systems have traditionally suffered from large cavity losses, a small tuning range, or severe mode hopping. The above mentioned N. Park et al. paper discloses an all fiber, single-frequency ring laser configuration using two fiber Fabry-Perot cavity filters, one narrow-band and one broad-band, to provide narrow bandpass wavelength filtering for stable and tunable laser operation which completely suppressed mode hopping.
This ring laser configuration used a temperature compensated, electronically tunable broad-band fiber Fabry-Perot (FFP) filter with low insertion loss and high finesse (see C. M. Miller and F. J. Janniello, Electron. Lett. 26, 632, 1990) in tandem with a further narrow-band FFP. However, even though the insertion loss of the FFP's is "low," each FFP filter element adds an insertion loss of 2.5 dB to the ring laser and the required isolators between each FFP add additional losses of 1dB each.